This invention relates to actuators and mechanisms which can generate motion and force output. It relates more specifically to devices that use electromagnetic forces to generate actuator output. It relates to computer operated machines. It relates to machines that act as an interface between a human user and machines of all types, including computers and machine tools.
This invention relates to computer controlled machines, where a computer specifies the desired performance of a mechanism. The computer may use sensor feedback, where sensor measurement of a mechanism is used by the computer to control the mechanism. A computer may also use sensor-less, or open-loop control, where the computer controls the mechanism without sensor feedback. One such control method is open-loop stiffness control, where the stiffness of a mechanism is specified, without sensor measurement of force applied to or position of the mechanism.
Computer controlled mechanism have a wide variety of applications, including robotics, automatic machining, consumer products, and medical devices. In lieu of a computer control, actuators can be controlled from simple controllers, signals from other mechanisms, or directly by humans (or even animals).
A specific application of this invention is for actuated human interface devices. Many machines are controlled, either directly or indirectly, by a human operator. The interface through which the operator controls the machine and receives information from the machine should be as easy to use as possible. The user may input commands to, receive information from, and otherwise interact with such machines through various devices, such as a lever, joystick, foot pedal, mouse (having buttons and a tracking mechanism), exoskeleton, keyboard, touch screen, digitized pad or tablet, head mouse, haptic force reflecting mechanism, etc. In general the component that the user physically touches is referred to as an "interface member."
In certain instances it is desirable that the interface device be actuated so that forces can be applied by the mechanism onto the user. A system that accomplishes this is sometimes referred to as a "force reflecting" system or a "haptic" interface, because it relates to the human system of touch.
An actuated interface device can function as both an input and output device. The user may input signals into a computer by manipulating the interface device, and the computer may output signals by imparting force and motion onto the user through the interface device. Thus, an interface member may also be referred to as an output device, or a display, etc. The format of the input and output signals can be in terms of force and torque, and position and rotation (and their time derivatives including velocity and acceleration).
Force reflecting interfaces are surveyed and described in general by Burdea, Grigore, in Force and Touch Feedback For Virtual Reality, John Wiley & Sons, Inc., New York (1996).
One use of actuated human interface devices is for telepresence and in teleoperated systems. Telepresence is when a person or teleoperator uses technology to mediate interactions with a remote physical environment. In the master/slave configuration, the user manipulates a "master" input device in the user's local environment. There may be a "slave" robot, typically in a different, non-local environment, which moves in accordance to the user's manipulations. The configuration of the master device may or may not conform to some degree to the conformation of the slave device. Teleoperation is useful in applications where direct interactions might be impossible because of physical conditions which are hazardous to humans, for example working with radioactive waste, or working in an underwater environment a mile deep. Other physically impossible conditions might be related to physical scale, such as nanomanipulation of a molecule, or the macromanipulation of an enormous crane. An example of telepresence is remote surgery, in which a surgeon uses a force-feedback scalpel at one location connected to a robotic scalpel in a surgical suite at another location. The surgeon's locally generated forces are transmitted to a remote actuator, and the remote forces generated by that actuator in contact with the patient are "fed back" to the surgeon's hand held scalpel, creating an effective, telemanipulative operation.
Another application of actuated human interface devices is "virtual presence." In virtual presence human operators control and interact with "virtual" machines and environments, which are not physical, but rather are "embodied" or reside in a computer model. A virtual environment relates to an environment that bears some mapping to an actual physical instance of the environment. For instance, a computer representation of a real slave environment is considered herein to be a virtual environment that corresponds to the physical slave environment. Virtual presence may also be used for semi-autonomous control of interaction with physical objects. This might occur when communications lag time between a person and the remote environment is too long, such as when operating a remote device on the planet Mars.
One objective of an actuated interface is to increase the realism of human interaction with virtual representation of objects by expanding the scope of human sensation and perception to include physical characteristics such as interaction forces with an object; and for movable objects; heft and inertia. This increase in realism allows humans to perform tasks better by leveraging human motor skills, and a heightened experience related to the interaction.
Actuated interface devices can be used to convey general information to the user. The force interactions may not necessarily correspond to a remote slave environment, or to a virtual environment mapped from a physical environment. For example a force interaction may be used to indicate the misspelling of a word in a word processing program. Actuated interface devices may also be used in computer games, by providing force-feedback to the users.
This invention also relates to actuators and mechanisms with numerous Degrees of Freedom (DOF). Each rigid body may have up to six DOF including translation and rotation. Moreover, the interface mechanisms may have numerous rigid members or flexible members. Thus, the overall number of DOF of such a mechanism may be greater than six. For example a master arm may have a hand portion, with several fingers, each with several joints.
For actuators in general, and especially for actuated interface mechanisms, high fidelity is often an important design consideration. A high fidelity actuator will have an output that is as close as possible to the desired output. The fidelity of the output relates to both accuracy in magnitude and in timing. Accordingly, a high fidelity mechanism will have a high bandwidth, and a minimum time delay from the instance that an output is desired and when the actuator responds. To achieve a high fidelity in the magnitude of the output it is desirable to minimize detrimental friction and backlash, which are often inherent in systems with transmissions between the actuators and the interface member.
Much of the engineering design effort related to force-feedback has centered on reducing the costs of present force-feedback input devices and development of software for authoring haptic cues, rather than basic force-feedback actuator design. Many present force-feedback input device rely on traditional motor actuators and closed-loop control, despite their many limitations, such as backlash and limited bandwidth. Therefore, there is a need for a high fidelity, robust, and low-cost actuator for use in these applications.
Known actuators meet many current needs. Most known mechanisms actuate a single DOF with a single powered actuator. However, many of the limbs of humans and animals function by a balancing between two opposed actuators (flexor and extensor muscles), which can be energized independently or simultaneously. Moreover, the forces generated by human and animal muscle vary depending on the length of muscle extension. Conversely, many mechanical actuators are designed to generate a force output that remains constant despite variations in the actuator's position.
Some known devices are similar to biological muscles in that they incorporate actuators whose output is a function of position, in a configuration that balances two opposed actuators. For example the pneumatic actuators described by H. M. Paynter "Low-Cost Pneumatic Arthrobots Powered By Tug-&-Twist Polymer Actuators" Japan/USA Symposium on Flexible Automation, Volume 1 pp. 107-110, July 1996, achieve the biological advantage of simplified control of position. However these systems use pneumatic power, which has the disadvantage that they require a source of pressurized fluid or gas which makes the size of the device large and noisy, and thereby unsuitable for many environments.
Known devices that use a single actuator for each DOF can operate under a control scheme that controls the stiffness, the position, or both of the device. However, most such devices require position sensors, and then perform calculations based on the measured position of the device. Thus, they use closed loop position and/or stiffness controllers. Often the computational time necessary to operate such closed loop stiffness or position control renders the device sluggish, or unrealistic in feel, given the typical computational limitations of computers available for common applications. Thus, the bandwidth of stiffness control is rather low. Further, instability in the control can arise if the lag in the control loop is too great, causing loss of control. For many actuated user interface mechanisms, low bandwidth degrades the quality of the device, and the computation time of the feedback loop is a limiting factor in system performance (see Burdea chapter 8, cited above).
Many mechanisms and actuated user interfaces require performance tradeoffs to achieve the desired magnitude of force or torque output. Often such systems use rotating electric motors as the power source. The torque output of an electric motor is proportional to the diameter of the rotor, and thus high torque motors have a very large diameter. In many actuator applications, a high force or torque is required to be applied at a low speed, and over a limited range of motion. To achieve a high level of torque while avoiding the cost of a large electric motor, a small motor is often operated at a high speed and coupled to a transmission that increases the torque while reducing the speed. However, transmissions typically have disadvantages that degrade the system performance of controlling position, force, and stiffness. Geared transmissions have backlash due to gaps between the meshing gear teeth. When the motor reverses direction, the transmission output does not respond until the gear gap is closed, which results in a lag in the response and rough performance. Other types of transmissions use cables, yet these systems have disadvantages of cable stretch and need adjustment to remove cable slack to avoid backlash. Novel and expensive actuators have been designed for user input devices, in part to avoid the use of transmissions, see Burdea cited above.
Many actuators and mechanisms have a limited amount of travel, such as a given distance for linear actuators and a given amount of rotation for rotating actuators. When the actuator or mechanism reaches the end of its travel, it often contacts a mechanical stop. The impact force with the mechanical stop can damage the actuator and mechanism. In addition, if a device is a user input device, then hitting the stop can abruptly change the device sensation in an undesirable fashion. If the maximum actuator force near the limits of travel is not sufficiently large, then hitting the mechanical stops can occur frequently. Accordingly, to avoid detrimental contact with the travel limit it is desirable to have a large actuator force near the travel limit.
In many systems with rotary motors, the motors themselves do not have mechanical stops, but they are coupled to an output member that does have a limited range of motion. In such systems the motor force typically remains constant over the complete range of travel. Since the actuator force does not increase near the limits of travel, hitting mechanical stops may occur frequently. Increasing the overall force generated by the motor may be inefficient, since large forces may only be required near the limits of travel.
Another disadvantage of using rotary motors, is that it can be difficult to build compact multi-degree of freedom systems. Rotary actuators are often combined in series when building multi-degree of freedom mechanisms. In a series configuration, the motors closer to the base reference move both the mechanism linkages and the motors that are farther from the base. Since the base motors must move the weight of other motors, their power requirements and size are large. Cable systems are sometime introduced to place the motors in a stationary location. However, as the number of degrees of freedom increase, the cable routing can become very complex. Accordingly , there is a need for actuators that can be configured in compact multi-degree of freedom mechanisms.
One possible approach to building compact multi-degree of freedom mechanisms is to use actuators that generate linear motion, and combine them in a parallel fashion. Thereby, the weight of the actuators is not directly applied to other actuators, and smaller actuators can be used. Thus it is advantageous to have compact linear actuators. However, most electric powered actuators are rotary.
For many computer input devices, it is desirable that the interface device return to a set point, or home position, when the user releases the interface member. A computer input device that has force feedback capabilities, can be programmed to return to a set point. However, if the force feedback actuators expend energy in return-to-set-point operations, then the device may overheat, since the return-to-set-point occurs frequently. In addition, when the force feedback device is in a retail sales display, where there is no electric power, it is desirable that potential customers can feel a return-to-set-point behavior. Accordingly, it is advantageous that the actuator for a force feedback computer input device be able to return the device to a set point position without the expenditure of energy.
A known method of returning a computer input device to set point is to use mechanical springs. This method does not expend energy, yet it has disadvantages. In a force feedback device, the actuators would have to overcome the spring force to generate the desired output force. Moreover, in order to prevent a loose spring, the springs are often pretensioned such that a user must overcome a threshold force before the interface member begins to move. This configuration hinders the user from imparting high precision, low force level inputs. Moreover, the threshold force may impart a directional preference in multi-degree of freedom devices that hinders the user from imparting their desired direction of input. The spring preload can also increase the friction in the system, further hindering high precision user input.
Known mechanisms are often deliberately built with specific degrees of compliance (i.e. springiness). For example, the shock absorbers in an automobile absorb variations in road surface, and a robot gripper may be designed with specific degrees of compliance to accommodate misalignments in the parts being assembled. The degrees of compliance are often fine tuned to achieve the desired performance of the mechanism, including the natural frequency of the mechanism and the level of forces applied by the mechanism. Typically the compliance in these devices are determined by the material properties of the materials and springs used. In such mechanisms, changing the compliance level requires a mechanical change to the mechanism.
When the conditions of operation change it may be desirable to change the compliance of a mechanism. For example when a robot arm experiences high accelerations it may be desirable to increase the stiffness in the gripper to avoid undesirable vibrations and perturbations. However, when the robot arm slows down to perform fine motion during assembly, a higher level of springiness in the gripper may be desirable to enable the robot gripper to accommodate part misalignment. Moreover, different levels of compliance may de desired for different parts being assembled. If the compliance is generated from mechanical springs, then modifying the mechanism compliance requires changing the mechanical configuration, which is complex and requires additional actuators. Accordingly, there is a need for a simple method to modify the compliance of a mechanism, such as with electronic control.
Thus one of the objectives of the invention includes an actuated mechanism that can be controlled with electrical power, yet incorporates some of the advantages of biological musculoskeletal systems. It is also an objective to provide passive stability, and the ability for open loop position and stiffness control, which would facilitate high bandwidth performance. Another objective of the invention is to facilitate an actuated human interface device, with a high fidelity position and torque or force signal provided to the user. Another objective is to provide an interface that can generate a large output force or torque in a compact configuration, without the use of geared or cable transmissions and thereby avoid backlash and friction in the system. It is also an objective to generate large forces near the limits of travel without increasing the overall size of the actuator, thereby minimizing or preventing impacts with any mechanical stops of the mechanism. It is also advantageous to provide actuators that can be configured as a multi-degree of freedom mechanism in a compact manner by using linear actuators in parallel configurations. It is further an object to provide such a mechanism that automatically returns to a set-point without expending power. Another object of the invention is to control stiffness of the actuators electrically, so that mechanism compliance and natural frequency can be adjusted, without mechanically changing the apparatus.
Accordingly, for the foregoing reasons, there is a need for an actuator that can provide position and stiffness control without sensors for either, and which provides a stable set point even in the absence of power to the device, and can otherwise achieve the objectives identified above.